The present invention relates generally to a double-passage acousto-optical device, that is, an acousto-optical device through which light passes twice, and specifically to various optical filters, wavelength add/drop devices, and optical cross-connects that are constructed using the double-passage optical device and methods for using them.
In optical filtering, double-stage optical filters are advantageous, because their filtering performance is increased compared to a single-stage filter having the same characteristics. Acousto-optical filters are known that provide for the interaction between light signals, propagated in waveguides formed on a substrate of birefringent and photoelastic material, and acoustic waves propagated on the surface of the substrate. The acoustic waves are generated by suitable transducers and are initially supplied by radio frequency signals.
The resonant (phase-matched) interaction between a polarized optical signal and an acoustic wave produces a wavelength-selective polarization conversion of the signal, in other words, a change of the polarization from its transverse electric TE component to its transverse magnetic TM component, which are orthogonal to each other, and vice versa. Following this interaction with the acoustic wave, the polarization components undergo not only the conversion to the corresponding orthogonal components, but also a frequency shift whose absolute value is equal to the frequency of the interacting acoustic wave (and therefore equal to that of the applied radio frequency signal). The sign of the frequency shift is a function of the state of polarization and of the direction of propagation of the acoustic wave with respect to the optical wave.
In such acousto-optical devices, by controlling the frequency of the optical and acoustic waves it is possible to tune the spectral response curve of the devices, which makes them suitable for being used as switches and as optical filters of the signals in optical telecommunications networks with wavelength-division multiplexing. These tunable switches and filters allow the selection of the signals to be changed and, thus, to reconfigure a network, without altering the cabling of the components.
These acousto-optical devices also allow the switching and simultaneous selection of different signals or channels, if the acoustic wave propagating at the surface of the substrate is the superimposition of different acoustic waves. In fact, the switches execute the combined switching of the signals at the wavelengths corresponding to the simultaneously applied frequencies and the filters have a pass band corresponding to the set of different wavelength intervals, determined by the frequencies of the acoustic waves.
As an example, EP 768555A1 in its FIG. 1 illustrates a 2xc3x972 acousto-optical switch with a polarization independent response. The switch comprises a substrate 1 in a birefringent and photoelastic material, consisting of Lithium Niobate (LiNbO3). Substrate 1 includes two polarization selective elements 2 and 3 and a conversion stage 4. The two polarization selective elements 2 and 3 are formed by polarization splitters in an optical waveguide, each comprising respective central optical waveguide portions 5 and 6 and optical waveguide input and output branches 7, 8, 9, 10 for splitter 2 and 11, 12, 13, 14 for splitter 3, respectively. The input branches 7 and 8 of splitter 2 are connected to input ports 71 and 81 of the switch through respective connecting optical waveguides 70 and 80. The output branches 13 and 14 of splitter 3 are connected to output ports 131 and 141 of the switch through respective connecting optical waveguides 130 and 140.
In practice these acousto-optical filters according to the description in EP 768555A1 comprise a waveguide chip of a length of about 65 mm, with optical guides 71, 81 and 131, 141 spaced apart by about 250 xcexcm. Although FIG. 1 of EP 0768555 is not drawn to scale, optical guides 15 and 16 are typically spaced apart by about 270 xcexcm. This distance includes polarization beam splitters/combiners (PBS) 5, 6. The end-faces are usually slant-polished (about 6xc2x0) to avoid any back-reflection from the end-faces. The wafer on which the device is realized has a 3-inch diameter, this dimension fixing the maximum length of the substrate 1 to about 60-65 mm.
EP 814364A1 describes a double-stage acousto-optical waveguide device. FIG. 12 in EP 814364A1 shows a switch, or add/drop node comprising, in addition to a third polarization conversion stage 303, a fourth polarization conversion stage 403. The fourth polarization conversion stage 403 is connected to an input polarization splitter 404 and to an output polarization splitter 405. In turn the splitter 405 is connected to the polarization splitter 204 by means of the connecting branch 210 and to the lateral waveguide 255. The ports 19, 20, 21 and 22 are connected to the line. The polarization splitter 404 is connected to input ports 25 and 26 through which the signals to be added or subtracted are introduced and signals to be added or subtracted are also introduced through the ports 23 and 24.
U.S. Pat. No. 5,452,314 describes an acousto-optical tunable filter with a pair of electrodes on opposite sides of the waveguide. The patent discloses the use of a voltage source in which an applied electric field controls the birefringence of the filter, and a tunable laser incorporating such an acousto-optical tunable filter. Suitably adjusting the potential applied by the voltage source results in suppression of sidelobes, correction of asymmetric sidelobes, and compensation for physical variations in the waveguide.
U.S. Pat. No. 5,002,349 and EP 805372 describe single converter acousto-optical tunable filters. The ""349 patent discloses an acousto-optical converter that allows multiple stages of such converters so as to provide for two-stage zero-frequency shifted converters and filters, lasers using an acousto-optical filter as a tuning element, polarization-independent converters, and wavelength-division-multiplexing routing switches.
U.S. Pat. No. 5,611,004 discloses a polarization independent acousto-optical tunable filter (AOTF). The patent describes in its FIG. 6 an embodiment where two stages of signal filtering are realized with only one transducer 43 on the substrate 31. Additionally, polarizer beam splitters 40 and 41 and a Faraday rotator 65 are used in the optical chain. Two stages of filtering are realized by passing the incoming beam of light through the AOTF a first time, reflecting the beam of light off of a mirror 67 and then passing the beam of light through the same AOTF a second time. A band pass filtered representation of the original beam of light is obtained at a circulator output 71 of an optical circulator 69 located at the input of the embodiment.
EP Application 98118377.5 describes a double passage acousto-optical device including an acousto-optical filter having a first converter coupled between first and second optical ports, a second converter coupled between third and fourth optical ports, and an optical combination coupled between the second port and the third port and including an optical isolating element.
EP Application 97113188.3 describes an acousto-optical device including a substrate of a material capable of propagating a surface acoustic wave along a portion of the surface of the substrate, a transducer for generating the surface acoustic wave, an optical waveguide formed in a substrate, and an acoustic absorber surrounding the portion of the substrate.
U.S. Pat. No. 5,712,932 describes optical cross connects for routing optical traffic between transmission paths in a wavelength-division-multiplexed optical communication system. The cross-connect switches in the ""932 patent use Bragg grating filters.
Applicants have discovered that conventional double-stage acousto-optical devices require excessive substrate size and incur signal losses in their arrangement as add/drop multiplexers or wavelength selective cross-connects. The larger wafer size and additional connections and losses leads to unnecessary complexity and cost for wavelength selection using acousto-optical techniques.
Applicants have discovered that add/drop multiplexers and optical cross-connect switches that use acousto-optical devices can have smaller wafer size, shorter length, fewer optical fiber connections and better performance than that used in conventional devices.
In one aspect, an acousto-optical add/drop multiplexer consistent with the present invention includes an acousto-optical switch on a birefringent and photoelastic substrate, the acousto-optical switch having a first optical port coupled to a first polarization splitter, first acousto-optical polarization conversion region including a first optical waveguide branch optically coupled between the first polarization splitter and a second polarization splitter, second acousto-optical polarization conversion region including a second optical waveguide branch optically coupled between the first polarization splitter and the second polarization splitter, and second and third optical ports coupled to the second polarization splitter. The multiplexer Includes a first circulator having an input port a switch port coupled to the first optical port, and an output port. A reflecting device is coupled to the second optical port of the switch.
Preferably, the multiplexer further includes a second circulator having a filter port coupled to the third optical port, a drop port and an add port.
Preferably, the first polarization splitter has cross and bar transmission respectively for orthogonal polarization components of received light.
Preferably the second polarization splitter has cross and bar transmission respectively for orthogonal polarization components of received light.
In an embodiment, the multiplexer also includes a polarization-mode-dispersion compensator coupled between the reflecting device and the second optical port of the switch. Preferably, the polarization-mode-dispersion compensator is a birefringent element, such as one of a polarization-maintaining fiber, and a birefringent crystal. Attentively, the polarization-mode-dispersion compensator comprises a Faraday rotator or a quarter wave plate.
In an alternative embodiment, a first polarization-mode-dispersion compensator is coupled between the filter port of the second circulator and the third optical port of the switch, and a second polarization-mode-dispersion compensator is coupled between the switch port of the first circulator and the first optical port of the switch. Preferably, the first and second polarization-mode-dispersion compensators are one of a polarization-maintaining fiber and a birefringent crystal.
In another aspect, a wavelength selective optical cross-connect consistent with the present invention includes at least two acousto-optical switches, each including, on a birefringent and photoelastic substrate, a first polarization splitter, a wavelength-selective polarization conversion stage including first and second optical waveguide branches coupled between the first polarization splitter and a second polarization splitter, a reflecting device coupled to one arm of the second polarization splitter, and a circulator having an input port for receiving line channels, a switch port coupled to the first polarization splitter, and an output port. The cross-connect further includes an optical path coupling second arms of the second polarization splitters in the respective acousto-optical switches.
In yet another aspect, an acousto-optical waveguide device selective in wavelength consistent with the present invention includes a birefringent and photoelastic substrate, a wavelength-selective polarization conversion region including first and second acoustic waveguides and first and second optical paths, a first polarization splitter coupled between one end of the first and second optical paths and only a first optical interface for the device, and a second polarization splitter. The second polarization splitter has input arms coupled to an opposite end of the first and second optical paths, a first output arm coupled to a second optical interface for the device and a second output arm. The acousto-optical waveguide device further comprises a reflecting device coupled to the second output arm of the second polarization splitter.
In another aspect, a method of using an optical device includes providing a plurality of optical channels to an acousto-optical switch having a first polarization splitter and a polarization conversion stage connected between the first polarization splitter and a second polarization splitter, switching at least one of the optical channels to a first arm of the second polarization splitter and other of the optical channels to a second arm of the second polarization splitter, reflecting the other of the optical channels back through the switch via the second arm, adding to the first arm a new channel coinciding in wavelength with the at least one of the optical channels, and combining the new channel and other of the optical channels at an output of the switch coupled to the first polarization splitter.
According to a further aspect, a method of multiplexing optical channels according to the invention comprises the steps of: providing a line optical channel at a first wavelength to an acousto-optical switch having a first polarization splitter and a polarization conversion stage connected between the first polarization splitter and a second polarization splitter; switching said line optical channel to a first arm of the second polarization splitter; reflecting said line optical channel back through the switch via the first arm; adding to a second arm of the second polarization splitter a new channel at a wavelength different from said first wavelength; and combining the new channel and the line optical channel at an output of the switch coupled to the first polarization splitter.
According to still a further aspect, a method of dropping optical channels according to the invention comprises the steps of: providing a plurality of optical channels to an acousto-optical switch having a first polarization splitter and a polarization conversion stage connected between the first polarization splitter and a second polarization splitter; switching at least one of the optical channels to a first arm of the second polarization splitter and other of the optical channels to a second arm of the second polarization splitter; reflecting the other of the optical channels back through the switch via the second arm; dropping said at least one of the optical channels from said first arm of the second polarization splitter.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.